The present invention relates to implantable stimulation devices, e.g., cochlear prothesis used to electrically stimulate the auditory nerve, and more particularly to an electrode array employing dielectric partitions or members that may be used with such implantable stimulating devices.
A cochlear prosthesis provides sensations of sound for patients suffering from sensorineural deafness. It operates by direct electrical stimulation of the auditory nerve cells, bypassing the defective cochlear hair cells that normally transduce acoustic energy into electrical activity in such nerve cells. In addition to stimulating the nerve cells, the electronic circuitry and the electrode array of the cochlear prosthesis must perform the function of the separating the acoustic signal into a number of parallel channels of information, each representing the intensity of a narrow band of frequencies within the acoustic spectrum. Ideally, each channel of information would be conveyed selectively to the subset of auditory nerve cells that normally transmitted information about that frequency band to the brain. Those nerve cells are arranged in an orderly tonotopic sequence, from high frequencies at the basal end of the cochlear spiral to progressively lower frequencies towards the apex. In practice, this goal tends to be difficult to realize because of the anatomy of the cochlea.
After extensive research in many centers employing a variety of surgical sites and approaches for the implantation of cochlear electrode arrays, a consensus has generally emerged on the use of the scala tympani, one of the three parallel ducts that, in parallel, make up the spiral-shaped cochlea. The electrode array to be implanted in this site typically consists of a thin, elongated, flexible carrier containing several longitudinally disposed and separately connected stimulating electrode contacts, perhaps 6-24 in number. Such electrode array is pushed into the scala tympani duct to a depth of about 20-30 mm via a surgical opening made in the round window at the basal end of the duct. During use, electrical current is passed into the fluids and tissues immediately surrounding the individual electrical contacts in order to create transient potential gradients that, if sufficiently strong, cause the nearby auditory nerve fibers to generate action potentials. The auditory nerve fibers arise from cell bodies located in the spiral ganglion, which lies in the bone adjacent to the scala tympani on the inside wall of its spiral course. Because the density of electrical current flowing through volume conductors such as tissues and fluids tends to be highest near the electrode contact that is the source of such current, stimulation at one contact site tends to activate selectively those spiral ganglion cells and their auditory nerve fibers that are closest to that contact site.
The selectivity of stimulation at each site provides a means for conveying different sound perceptions. Typically, cells (and their corresponding auditory nerve fibers) that are in one region or area convey sound perceptions within a given frequency band or channel. This selectivity of stimulation at each site provides a lower limit on the useful spacing available between adjacent sites. That is, if adjacent sites closer than that lower-limit spacing are stimulated simultaneously, then the signals carried by the neurons can no longer distinguish respective frequency bands separately, but rather will convey signals that are contaminated by cross-talk between channels and may be perceived as unclear and/or excessively loud. This lower-limit spacing also effectively limits the maximal number of parallel channels of information that can be conveyed about acoustic signals such as speech because the length of the scala tympani over which the complete range of speech signal frequencies is represented is fixed at about 15-20 mm in length. Further, the actual selectivity of stimulation at each site is limited by the spreading of the injected electrical current through the volume-conductive tissues and fluids of the cochlea.
Several stimulation strategies have been described in the prior art for maximizing the selectivity. These include:
Bipolar Stimulation--Bipolar stimulation provides two closely spaced electrode contacts within the scala tympani which are used to provide both a source and sink for the stimulating electrical current (see e.g., U.S. Pat. No. 4,819,647), instead of the monopolar configuration in which the sink for all channels is a common electrode located outside of the cochlea. With bipolar stimulation, the rate at which the current density decreases with distance from the electrodes is much greater than with monopolar stimulation. There is a significant disadvantage, however, in that the amount of current required to produce adequate stimulation at each site and the power required to pass that current through the tissues is much higher than for monopolar stimulation. This is a significant disadvantage for the efficient design and operation of implanted microminiature circuitry in a portable battery-powered system.
Directional Contacts--In some electrode designs, the individual contacts are shaped like cigar bands, causing the stimulating current to radiate in all directions equally. By using smaller contacts that occupy only a portion of the transverse crosssection of the electrode carrier at a particular longitudinal position, the current density can be made asymmetrical (see, e.g., U.S. Pat. Nos. 4,686,765; 4,819,647). If the design of the electrode array and its placement by the surgeon permits the contacts to be reliably positioned so as to be facing the medial wall of the scala tympani, in which the spiral ganglion cells reside, the selectivity will be somewhat improved. The improvement tends to be limited, however, by the tendency of stimulating current to disperse broadly through the relatively conductive fluids of the scala tympani. Furthermore, the small surface area of the contacts will increase their electrical impedance and, hence, the voltage required to deliver a particular stimulating current.
Spiral-Shaped Carriers--Regardless of the design of the electrode contacts and the tissues in which they reside, the current density is always highest nearest the contact surface. One strategy, therefore, that has been used with small contacts that face the medial wall is to embed them in an elastomeric carrier that is molded into the shape of the cochlear spiral (see U.S. Pat. Nos. 4,686,765; 4,819,647). Upon insertion, the carrier regains its spiral shape, drawing the contacts close to the medial wall. The fabrication of such an electrode array is somewhat complicated, however. Furthermore, special instruments and techniques must be used by the surgeon in order to hold the electrode straight in order to effect insertion into the round window opening.
Space-Filling Carriers--Yet another technique known in the art to position directional contacts near the medial wall is to make the electrode array relatively thick in cross-section. This can be done by using a mold whose dimensions are sized closely to the cross-sectional dimensions of the scala tympani (see U.S. Pat. Nos. 4,686,765; 4,819,647). Other techniques that achieve this same purpose could include adding flexible fins along the lateral edge to push the electrode towards the medial wall, or by making some or all of the carrier from a material that swells, inflates, or otherwise changes its dimensions after insertion. One problem with these techniques is that there is a fairly large range of variability in the dimensions of the scala tympani from one patient to another and there are often irregularities in cross-sectional area along the length of an individual scala tympani. As the electrode contacts get closer to the medial wall, even small fluctuations in the actual gap and the points of actual contact with the side walls can cause large changes in the distribution of the stimulating currents from each site, which may disrupt the orderly tonotopic representation and the balance of loudness between channels. Furthermore, the surgeon who performs the implant generally prefers an electrode that is as thin as possible to improve the chances of being able to insert it successfully in any conditions that may obtain.
Separate Contact Placements--Another technique for maximizing the selectivity of stimulation sites is to drill into the scala tympani through its lateral wall at multiple locations and place a separate stimulating electrode in each site, as described by Chouard and MacLeod (1976). Before sealing the holes, small plugs of a nonconductive material such as silicone elastomer are inserted into the holes so as to flank each electrode in an attempt to prevent its currents from spreading longitudinally in the conductive fluid of the scala tympani. Several problems developed with this technique that caused it to be abandoned. Only one side of the cochlear spiral is surgically accessible in this manner and even then, it is difficult to perform the multiple fenestrations without damaging the extremely delicate membranes that separate the three parallel canals. Further, even if accurately-sized plugs could be installed, they tend to block only longitudinal conduction and not lateral conduction out of the scala tympani and into adjacent, overlying turns of the spiral; in fact, the scar tissue that eventually seals over the fenestrations tends to be more conductive than the bone that it replaces, actually channeling stimulation currents laterally rather than in the desired medial direction.
Increases in the numbers of channels and the rates of stimulation in cochlear implants have exceeded the information carrying capacity of currently available electrode arrays. Cross-talk of stimulation between adjacent electrode sites limits the number of useful channels, particularly if more than one channel must be stimulated simultaneously to achieve reasonable repetition rates with power-efficient pulse widths. Furthermore, design features now available to improve channel selectivity (radial bipolar contact geometry, medial wall-hugging curvature and space-filling profile) also make the electrode more difficult to manufacture and to implant.
It is evident, therefore, that improvements are needed in cochlear electrodes to address the above and other concerns.